Multi-function antenna coupler

ABSTRACT

The antenna coupling circuit facilitates simultaneous reception and transmission of information carrying radio signals through a common antenna. A single filter having a plurality of helical resonator sections forms the harmonic filter of the transmitter, the preselector of the receiver, and a duplexer for connecting both the transmitter and receiver to a single antenna. A first plurality of the helical resonator sections are connected between the antenna and the input stage of a receiver which passes the band of frequencies to be received while reflecting the frequencies to be transmitted. A second plurality of helical resonator sections are connected between the output of a transmitter and the antenna which passes the frequencies to be transmitted while reflecting the frequencies to be received. The coupling between individual helical resonators is arranged to discriminate against frequencies higher than the passband. All of the cells of the filter circuit are mechanically connected together to form an integral compact structure.

United States Patent [191 Choi et al.

[ 1 Apr. 17, 1973 MULTI-FUNCTION ANTENNA COUPLER [75] Inventors: CharlesChoi, Hoffman Estates; Maynard H. McGhay, Schaumburg, both of I11.

[73] Assignee: Motorola, Inc., Franklin Park, Ill.

[22] Filed: July 2, 1971 [21] App]. No.: 159,194

Primary Examiner-Benedict V. Safourek Att0rney-Mueller & Aichele 5 7]ABSTRACT The antenna coupling circuit facilitates simultaneous receptionand transmission of information carrying radio signals through a commonantenna. A single filter having a plurality of helical resonatorsections forms the harmonic filter of the transmitter, the preselectorof the receiver, and a duplexer for connecting both the transmitter andreceiver to a single antenna. A first plurality of the helical resonatorsections are connected between the antenna and the input stage of areceiver which passes the band of frequencies to be received whilereflecting the frequencies to be transmitted. A second plurality ofhelical resonator sections are connected between the output of atransmitter and the antenna which passes the frequencies to betransmitted while reflecting the frequencies to be received. Thecoupling between individual helical resonators is arranged todiscriminate against frequencies higher than the passband. All of thecells of the filter circuit are mechanically connected together to forman integral compact structure.

7 Claims, 6 Drawing Figures MULTI-FUNCTION ANTENNA COUPLER BACKGROUND OFTHE INVENTION Many types of communication apparatus include bothtransmitters and receivers which either simultaneously or individually,operate on adjacent frequencies. Mobile, portable and hand-heldapparatus of this type frequently each employ a single, common antennawhich accommodates both transmission and reception. It is usuallynecessary in these applications to provide one coupling network(preselector) between the antenna and the input of the receiver andanother coupling network (harmonic filter) between the antenna and theoutput of the transmitter. These networks allow the transmitted signalto flow to the antenna and the received signal to flow into thereceiver.

In the past, a plurality of different kinds of filtering networks haveoften been included in a single communication apparatus to facilitateduplex operation. For instance, a duplex filter arrangement mightinclude a duplexer connected to an antenna, a preselector connectedbetween the duplexer and a receiver, and a harmonic filter connectedbetween a transmitter and the duplexer. Furthermore, trap circuits areoften respec tively utilized in cooperation with the harmonic filter andthe preselector to reject the received frequency and transmittedfrequency by dissipation. Sometimes this prior art combination employsdifferent kinds of filters having both lumped and distributedcomponents, such as transmission lines, in a single communicationsystem. Transmission lines and connectors are used to form the signalflow paths between the various component filters of the prior artantenna coupling systems.

These prior art networks have been found to have disadvantages incommunication systems which must take up limited amounts of space andwhich are required to have high efficiencies. Provision for a pluralityof different types of filter, their connecting cables and connectorsoften require an excessive amount of space. Moreover, the connectors,connecting cables and dissipating trap circuits have losses inherentwhich reduce the overall efficiency of the communications system.Moreover, particularly in commercial products where cost must be kept toa minimum, the manufacturing costs of prior art antenna coupling andfiltering systems have been excessive. Furthermore, some networksemploying a plurality of different types of filters have been found tobe unreliable and difficult to align. Also, prior filters haveundesirable spurious frequency responses.

SUMMARY OF THE INVENTION One object of this invention is to provide amultifunction filter network for allowing operation of a transmitter anda receiver through a common antenna, and which is inexpensive, reliableand requires a minimum amount of space.

Another object of the invention is to provide a single compact filterstructure for a radio transmitter and receiver, comprised of integral,homogeneous cells which facilitate easy and inexpensive construction.

Still another object of the invention is to provide a multifunctionantenna coupling network which is easy to accurately align at high levelsignals.

A further object of the invention is to provide an efficient antennacoupling circuit having a first leg which passes information bearingsignal to be transmitted and reflects all other signals within asubstantial range above and below the center frequency of thetransmitted signal and a second leg which passes a received signal andreflects, all other signals within a substantial range above and belowthe frequency of the received signal.

A still further object of the invention is to provide a multi-functionantenna coupling circuit, for selectively coupling a common antenna to areceiver and transmitter, which utilizes helical resonators but whichprovides a greater degree of attenuationto frequencies above itspassband than conventional helical resonator filters.

The antenna coupling circuit facilitates simultaneous reception andtransmission of radio signals through a common antenna. A first leg ofthe circuit includes a first plurality of helical resonator cellsforming an input stage of a receiver, which is connected between theantenna and a subsequent stage of a receiver, and a second leg includesa second plurality of helical resonator cells forming an output stage ofa transmitter, which is connected between another stage of thetransmitter and the antenna. All of the cells of the filter aremechanically connected together to form an integral structure. Lumpedcapacitors are connected between the antenna terminal and endmosthelical windings of the transmitter and receiver legs, to facilitatereflection rather than dissipation of the frequencies rejected by eachleg. Each of these capacitors utilize dielectric material affixed to awinding and a conductor affixed to the dielectric material to form aninexpensive capacitor structure. Apertures between cooperating cells arearranged to allow capacitive coupling therebetween to facilitate maximumattenuation of spurious responses having frequencies greater than thehighest frequency in the passband. Tuning elements for both thetransmitter and receiver portions pass through a common member andfacilitate simultaneous tuning adjustment of both legs of the filter athigh power levels.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram showing theconnection of I a multi-function antenna coupling circuit of onearrangement between a stage of a receiver, a stage of a transmitter, andan antenna;

FIG. 2 is an approximately full scale perspective view of a partiallyexploded, multi-function coupling circuit having another arrangement;

' FIG. 3 is a broken away view of an enlarged portion of the structureof the coupler of FIG. 2 showing the connection of the antenna terminalto the endmost resonators of the transistor and receiver legs;

DETAILED DESCRIPTION Referring now to FIG. 1, a block diagram of aduplex communication system is shown which facilitates simultaneoustransmission and reception of information bearing radio signals throughantenna 12. Multifunction antenna coupler or filter circuit 14, whichincludes a plurality of cells mechanically arranged in anL-configuration, is comprised of first and second reflective narrow bandfilter legs and 21. Antenna coupling network 14 performs signalselecting functions previously performed by the combinationof aduplexer, preselector and harmonic filter. Receiver filter 15, which maybe considered to be the input stage of the receiver, is connectedbetween antenna port 16 and input terminal 18 of receiver 20, andtransmitter filter 21, which may be considered to be the output stage ofthe transmitter, is connected between output terminal 22 of transmitter24 and port 16. Receive leg 15 includes series connected cells 26, 28,30, 32 and 34; and transmit leg 21 includes series connected cells 36,38, 40 and 42. Each cell is a high-Q shunt helical resonator which has anarrow passband characteristic at high frequencies.

Input terminal 18 of receiver is connected to a first input of mixer 46,output terminal 48of local oscillator 50 is connected to a second inputof mixer 46. Output 52 of mixer 46 is connected to an intermediatefrequency (I.F.) amplifier and discriminator 54. Audio amplifier 56 isconnected between the output of block 54 and loudspeaker 58. Althoughthe block diagram of a frequency modulation (FM), superhetrodynereceiver has been shown and described, a receiver of any of the knowntypes could be advantageously employed with circuit 14.

Transmitter 24 includes microphone 60 which is connected to the input ofpreamplifier 62. The input of frequency modulated oscillator 64 isconnected to the output of preamplifier 62. Exciter 66 is connectedbetween the output of frequency modulated oscillator 64 and the input ofmultiplier 68. Power amplifier 70 is connected between the output ofmultiplier 68 and the input port of transmitter filter leg 21. Althougha block diagram of a FM transmitter has been shown and described, otherknown types of transmitters could be advantageously employed in placethereof.

In operation, if antenna 12 receives a signal falling within the signalpassband of filter 15, filter cells 26 through 34 form a preselectorwhich passes the received signal to input 18 of mixer 46. Receiverfilter leg 15 also reflects, and thereby attenuates, all signals passedfrom input 16, to input 18 having frequencies outside of its passband,including signals being developed by transmitter 24. Filter cells 36through 42 reflect signals received by antenna 12 which are outside ofits passband so that they are not developed at transmitter outputterminal 22. Otherwise, these received signals could interfere with theoperation of transmitter 24. Since signals having frequencies within thepassband of receiver filter 15 are reflected rather than dissipated byfilter 21, desired received signals being induced in antenna 12 areefficiently applied to receiver input 18. Similarly, since thetransmitter signals passing through filter 21 are reflected rather thandissipated by filter'15, the transmitted signals are efficiently appliedto antenna 12. Furthermore, filter cells 36' through 42 forma harmonicfilter to pass only the desired components of the modulated outputsignals developed by transmitter 24 so that virtually no unwantedinterference is created on adjacent channels. Although the cells offilter 14 are arranged in an L shape, there are many other possible waysin which they could be arranged to facilitate utilization of availablespace within the structure of a particular product. Each of the cells ofthe filter of FIG. 1 can be constructed in a manner similar to the cellstructure shown in FIG. 2.

A partially exploded, perspective view of multi-function filter 84,which includes helical resonator cells arranged in a rectangularconfiguration, is shown in FIG. 2. Filter 84 of FIG. 2 is comprised of aconductive housing 86 having rectangular side walls and end walls. Flatrectangular top plate 88 and flat rectangular bottom plate 90, which canbe seen in FIG. 3, are adapted to be attached to housing 86. Theinterior space of housing 86 is divided into a plurality of separatecells by a longitudinal partition 92 and a plurality of transversepartitions, e.g., 94, which are perpendicular to partition 92 and to theside walls of housing 86. Each cell has a housing member formed byassociated parts of the partitions, the end walls, side walls and thetop and bottom plates.

Each of the cells includes a coil form, e.g., 100, which is made from aninsulating or dielectric material such as polystyrene plastic. Aconductor, as shown in FIGS. 3 and 6, is wound on each coil form toprovide a helical winding or coil. The inner surfaces of each cellfacing each coil are comprised of or coated with a conductive material.Apertures or windows e.g., 103, are selectively provided in selectedones of the transverse partitions to allow electromagnetic couplingbetween cooperating cells. Top plate 88 and bottom plate may be fastenedto the housing by screws 107.

Each of the coil forms, e.g., 100, has a cylindrical hole 106 which runsalong a longitudinal axis thereof and intersects the end of the coilform facing the underside of top plate 88. As later explained in greaterdetail, each of these cylindrical holes receives an associated tuningelement, e.g. 108, when top plate 88 is mounted onto housing 86. Radiofrequency (RF) connectors 1 12, 114 and 116 respectively, facilitateelectrical connection to antenna 12, the output of transmitter 24 andthe input of receiver 20. Cells 118, 120, 122 and 124 form a transmitterfilter leg of antenna coupler 84 and cells 126, 128, 130, 132, 134 and136 form a receiver filter leg. The antenna coupling network of FIG. 2is comprised of integral homogeneous cells which facilitate easy andinexpensive construction of both legs of the filter.

Corresponding parts of multi-function antanna coupler 84 are referred toby the same reference numbers and in FIGS. 2, 3 and 6. Each helical coilcooperates with the inner conductive walls of the housing and plateswhich surround it to form a resonant chamber or shunt helical resonator.The electrical characteristics of each helical resonator are similar tothose of a very high'Q parallel resonant circuit. Therefore, each filterleg includes a plurality of cooperating resonators which would provide avery high impedance to signals applied directly thereto which havefrequencies near its resonant frequency. Thus, a helical resonatorfilter generally passes signals having frequencies which aresubstantially equal to its resonant frequency and reflects frequencieswhich are substantially unequal to its resonant frequency. Thisdesirable result is obtained by inserting lumped capacitorsrespectively, between input cell 126 of the receiver leg and antennaterminal 112 and between output cell 118 of the transmitter leg andantenna terminal 112.

Referring now to FIG. 3, portions of side wall 138, transverse partition140, and transverse partition 142 are broken away to more clearly showthe capacitive coupling between antenna connector or terminal 112 andthe helixes of cells 118 and 126. Partition 140 is a solid wallextending from longitudinal partitions 92 to side wall 138. Helicalwinding 142 of cell 126 is formed from copper or other conductive wirehaving a circular configuration. End 144 of winding 142 is mechanicallyand electrically connected to conductive base plate 90 thereby forming alow impedance end adjacent to base plate 90. Winding 142 lies in ahelical groove molded into nonconductive coil form 146. High impedanceend 147 of conductor 142 extends away from coil form 146 toward cell118. Tubular sleeve 148, which is formed from nonconductive ordielectric material, is inserted over end 147 of conductor 142. Curvedconductor 149 includes: first end 150 which is glued or otherwiseattached to nonconductive sleeve 148, an intermediate portion 152 whichpasses through a small insulated hole in partition 140, (not shown) anda second end 156 which is connected to a center piece of RF connector112, which is shown in FIG. 3. End 147 of helical coil 142,nonconductive material 148 and end 150 of curved conductor 152respectively, form a first plate, a dielectric and a second plate for afirst lumped capacitor connected between the antenna connector and theinput cell 126 of the receiver leg of filter 84. In a similar manner,conductor 170 forms a first plate, insulating sleeve 172 forms adielectric and end 173 of conductor 174 forms a second plate of a secondlumped capacitor coupling the antenna connector to output cell 1 18 ofthe transmitter leg of filter 84.

Since the two above described coupling or impedance inverting capacitorsare included within the structure of filter 84 no additional space isrequired for them. Moreover, the capacitors are inexpensive and easy tofabricate. They may be adjusted by selectively cutting away portions ofeither the first or the second plates. If desired, the capacitors couldbe formed by portions of the helixes other than the ends thereof.

Helical resonators generally have undesired spurious responses at oddmultiples of their resonant frequencies. The coupling betweencooperating helical resonators is arranged to provide maximumattenuation to spurious responses having frequencies in excess of thedesired passband of each leg of the network. Rectangular coupling insert162 of FIG. 3, is comprised of a thin sheet of conductive materialhaving an aperture 160 located therein. Tracks are provided inlongitudinal partition 92 so that insert 162 may be slid in and out tofacilitate adjustment therein, if desired. Alternatively, insert 162could be integrally molded into longitudinal partition 92. As cell 126is energized by a received signal having a frequency within itspassband, magnetic fields are generated at end 144 of helical conductor142 and electric fields are generated at end 147 thereof. Aperture orwindow is located closer to end 147 of the helical coil than to end 144.Therefore, aperture 160 facilitates predominately electric field orcapacitive coupling between cell 126 and cell 128. If the position ofinsert 162 was reversed so that aperture 160 was located near base plate90, the coupling between cells 126 and 128 would be predominantlythrough magnetic field or inductive coupling.

Increased attenuation to spurious frequencies in excess of the passbandof a filter including a plurality of cooperating helical resonators hasbeen discovered to result from the use of capacitive coupling ratherthan inductive coupling. This beneficial result is believed to stem fromthe observation that the capacitive coupling coefficient betweenadjacent helical resonator cells decreases more rapidly than theinductive coupling coefficient in response to increasing frequency. Morespecifically, FIG. 4 shows the relation between the square of theinductive coupling coefficient and the square of the capacitive couplingcoefiicient as a function of the radial phase constant. Ordinate axes183 is marked off with a logarithmic scale representing the square ofthe respective coupling coefficients and abscissa axes 184 is marked offin a linear scale representing radial phase constant which increases asthe input or driving frequency increases. Curves 185 and 186 arerespective plots of the square of the capacitive coupling coefficientand the square of the inductive coefficient. Since the square of thecapacitive coupling coefficient, as shown by the slope of curve 185,decreases more rapidly than the square of the inductive couplingcoefficient, as shown by the slope of curve 186, with an increase infrequency, helical resonators utilizing capacitive coupling providegreater attenuation to high frequency spurious signals than resonatorsutilizing inductive coupling.

A multi-function antenna coupler similar in mechanical form to the oneshown in FIG. 2, and having a length of approximately 5 inches, a widthof approximately 2 inches and a depth of approximately 2 inches has beenbuilt and tested for operation on the very high frequency (VI-IF)commercial communication band between 132 and 174 megahertz (MHz). Eachcell of this filter has a square transverse cross-section having sideswhich are about 1% inches long. The coupling aperture is about 1% incheslong and 0.2 inch wide. The insertion loss verses frequencycharacteristic for this antenna coupler is shown in FIG. 5. Solid curve188 represents the insertion loss characteristic of the receiver filterleg and dashed curve 189 represents the insertion loss characteristicsfor the transmitter filter leg. Ordinate axis 190 is marked off indecibels (db) and abscissa 191 is marked off with a logarithmic scalebeginning at 100 MHz and extending to 500 MHz. As shown by curve 188,the receive leg of the filter provides little attenuation, e.g., about4.5 db, to a received signal having a center frequency of about l50 MHzbut it provides a large insertion loss, e.g., about 82 db, to signals atmost other frequencies including the transmitter center frequency of 155MHz. Similarly, as shown by characteristic 188, the transmit legprovides little insertion loss to frequencies close to the transmittercenter frequency, e.g. 1.5 db, but it provides a relatively largeinsertion loss, e.g. 80 db, at most other frequencies.

As shown in FIG. 5, third harmonic inherent spurious response orpassband 192, which is centered at about 450 MHz, occurs in the receiveleg, and inherent spurious passband 193 which is centered at about 465MHz, occurs in the transmit leg. Undesired signals occurring within thefrequency ranges of these spurious passbands tend to be passed through afilter to a greater extent than undesired signals having otherfrequencies outside of the desired passband. Since there is an ultrahigh frequency (UHF) commercial band located between 406 MHz and 512,there is a strong likelihood that signals at the frequencies of thespurious passbands will be applied to the filter. The novel arrangementof apertures in filter 84, which employs capacitive coupling betweenadjacent resonators rather than inductive coupling, in at least dbattenuation to signals within the spurious passbands in each leg of theantenna coupler. Prior art helical resonators having apertures openingto the low impedance end of the helical windings, present attenuationsof only about 7 db to the undesired third harmonic signals. Thus, themulti-function antenna coupling circuit, which selectively couples acommon antenna to a receiver and transmitter, provides a greater degreeof attenuation to undesired frequencies above its passband thanconventional helical resonator filters.

FIG. 6 is an end section view of output cell 136 of the receiver filterleg of antenna coupler 84. Top plate 88 is shown in its position onhousing 86. Screw 194 extends through base-plate 90 to mechanicallyfasten coil form 195 thereto. End 196 of conductor 197 is electricallyand mechanically connected to base plate 90 by solder 198. Conductor 199is connected between a selected point on end 196 and a center conductor200 of receiver RF connector 116. The precise point of connection ofconductor 199 to coil 197 may depend upon the necessary impedancetransformation for maximum power transfer from the filter into thereceiver. The coupling from the transmitter output, which is connectedto the center conductor of connector 114, into cell 124 may also beaccomplished by utilizing a similar structure. Also, in someapplications it might be desirable to include impedance transformingcapacitors between either or both of RF connectors 114 and 116 and therespective coils of cells 124 and 136.

Tuning device 202, which is typical of the tuning devices associatedwith the other cells of the multifunction antenna coupler, is associatedwith cell 136. It may be formed from a threaded rod 204 having a slot206 in one end thereof. Threaded opening 208 in top plate 88 receivestuning device 202 which extends into aperture 210 of coil form 195. Theportion of adjustable member 202 extending below top plate 88 and intothe interior of cell 136 forms a first plate of an adjustable tuningcapacitor, endmost winding 212 of helical conductor 197 forms a secondplate, and the portion of coil form 195 located between these two platesacts as a dielectric. As member 202 is turned by a blade inserted inslot 206 so that it extends either more or less into aperture 210, theeffective surface area of the first plate is varied thereby changing theequivalent capacitance. The resonant frequency of the cell is therebychanged or adjusted. Thus, by adjusting the other tuning members similarto member 202 associated with each cell the resonant frequencies of thetransmitter and receiver legs of the antenna coupling circuit isdetermined. The resonant frequency or center of the passband of thereceiver leg may be selected to be either greater than or less than theresonant frequency or center of the passband of the transmitter leg.Moreover, the tuning structure facilitates accurate alignment of themulti-function antenna coupler at high signal levels whereas prior artantenna couplers, including a plurality of different types of elements,often can.

only be adjusted at lower signal levels. The feature of antenna filter86 enabling high power adjustment is particularly desirable with respectto the transmit leg, because it may be difficult to adjust thetransmitter so that it generates a low level signal.

What has been described, therefore, is a multi-function filter networkfor allowing duplex operation through a common antenna between atransmitter and a receiver, which is inexpensive, reliable and whichrequires a minimum amount of space. The filter is comprised of integralhomogeneous cells which facilitate an inexpensive construction.Selective positioning of the apertures between the cells cause insertionloss characteristics indicating a greater attenuation to frequenciesabove the passbands of the filter legs than heretofore realized withconventional helical resonator filters having the same number of cells.Furthermore, the antenna coupler can be easily and quickly aligned evenwith high level signals passing therethrough.

We claim:

1. Filter apparatus for selectively coupling signals from an antenna toan input stage of a radio receiver operating on a first frequency bandand from an output stage of a transmitter operating on a secondfrequency band to the antenna, the filter apparatus including incombination:

a conductive housing including a plurality of conductive enclosures andhelical windings in each of said enclosures forming a plurality ofhelical resonators;

a first group of said helical resonators being tuned to the firstfrequency band to form a receiver preselector filter having an outputterminal adapted to be connected to the input stage of the radioreceiver for selecting signals applied thereto;

a second group of said helical resonators being tuned to the secondfrequency band to form a transmitter harmonic filter having an inputterminal adapted to be connected to the output stage of the transmitterfor selecting only the desired components of the signal therefrom;

an antenna terminal on said housing adapted to be connected to theantenna;

first circuit means coupling said winding of said receiver filter tosaid antenna terminal whereby the receiver filter selects signals of thefirst frequency band and applies the same to the receiver stage andrejects signals of the second frequency band; and 4 second circuit meanscoupling said winding of said transmitter filter to said antennaterminal whereby the transmitter filter selects signals of the secondfrequency band from the transmitter stage and applies the same to theantenna terminal and rejects signals of the first frequency band.

2. The filter apparatus of claim I wherein said first circuit meansincludes capacitor means having a first plate connected to said antennaterminal and a second plate connected to said receiver filter.

3. The filter apparatus of claim 1 wherein said second circuit meansincludes capacitor means having a first plate connected to said antennaterminal and a second plate connected to said transmitter filter.

4. The filter apparatus of claim 1 further including:

first capacitor means having a first plate formed by a portion of saidwinding of said receiver filter, a dielectric formed by a firstinsulating member which is mechanically connected to said winding ofsaid receiver filter, and a second plate formed by a first conductivemember having first and second ends, said first end being connected tosaid first insulating member and said second end being connected to saidantenna terminal; and

second capacitor means having a first plate formed by a portion of saidwinding of said transmitter filter, a dielectric formed by a secondinsulating member which is mechanically connected to said winding ofsaid transmitter filter, and a second plate formed by a secondconductive member having first and second ends, said first end of saidsecond conductive member being connected to said second insulatingmember and said second end of said second conductive member beingconnected to said antenna terminal.

5. The filter apparatus of claim 1 wherein said receiver filter includesa plurality of cooperating helical resonators which tend to have aspurious passband, each of said resonators having a helical coil withfirst and second ends and a longitudinal axis, and a housing memberenclosing said helical coil which is formed by four conductive sidewallsand first and second conductive end walls, said longitudinal axis ofeach of said coils being perpendicular to said first and secondconductive end walls;

said resonators being juxtapositioned with respect to each other so thatsaid housing member of each resonator has a common sidewall with thehousing member of a cooperating resonator;

said first end walls of said resonators being formed by a singleintegral conductive member;

means electrically connecting said first end of each of said coils tosaid integral conductive member; and

each of said common sidewalls having an aperture formed therein, saidapertures being located adjacent to said second ends of said coils tofacilitate predominantly capacitive coupling between said adjacentcooperating resonators to provide attenuation to signals havingfrequencies lying within said spurious passband.

ill

6. The filter apparatus of claim 1 wherein said transmitter filterincludes a plurality of cooperating helical resonators which tend tohave a spurious passband, each of said resonators having a helical coilwith first and second ends and a longitudinal axis, and a housing memberenclosing said helical coil which is formed by four conductive sidewallsand first and second conductive end walls, said longitudinal axis ofeach of said coils being perpendicular to said first and secondconductive end walls;

said resonators being uxtaposmoned with respect to each other so thatsaid housing member of each resonator has a common sidewall with thehousing member of a cooperating resonator; said first end walls of saidresonators being formed by a single integral conductive member;

means electrically connecting said first end of each of said coils tosaid integral conductive member; and

each of said common sidewalls having an aperture formed therein, saidapertures being located adjacent to said second ends of said coils tofacilitate predominantly capacitive coupling between said adjacentcooperating resonators to provide attenuation to signals havingfrequencies lying within said spurious passband.

7. The filter apparatus of claim 1 wherein each of said resonators has ahelical coil with first and second ends, a coil form for supporting saidcoil and having first and second ends and an aperture extending fromsaid second end into said coil form, and a housing member formed by fourconductive sidewalls and first and second conductive end walls;

said resonators of said receiver filter being juxtapositioned so thatsaid housing members thereof have common intermediate sidewalls;

said resonators of said transmitter filter being juxtapositioned so thatsaid housing members thereof also have common intennediate sidewalls;

said first end walls of said housing members of each of said filtersbeing formed by a first integral conductive member, means electricallyconnecting said first ends of said coils to said first conductivemember;

said second end walls of said housing members of each of said filtersbeing formed by a second integral conductive member having a surfacelocated in a plane which is parallel to and adjacent to said second endsof said coil forms; and

a plurality of tuning means each extending through said secondconductive member into said aperture of one of said coil forms, saidplurality of tuning means facilitating simultaneous adjustment of saidresonant frequencies of said receiver and transmitter filters.

1. Filter apparatus for selectively coupling signals from an antenna toan input stage of a radio receiver operating on a first frequency bandand from an output stage of a transmitter operating on a secondfrequency band to the antenna, the filter apparatus including incombination: a conductive housing including a plurality of conductiveenclosures and helical windings in each of said enclosures forming aplurality of helical resonators; a first group of said helicalresonators being tuned to the first frequency band to form a receiverpreselector filter having an output terminal adapted to be connected tothe input stage of the radio receiver for selecting signals appliedthereto; a second group of said helical resonators being tuned to thesecond frequency band to form a transmitter harmonic filter having aninput terminal adapted to be connected to the output stage of thetransmitter for selecting only the desired components of the signaltherefrom; an antenna terminal on said housing adapted to be connectedto the antenna; first circuit means coupling said winding of saidreceiver filter to said antenna terminal whereby the receiver filterselects signals of the first frequency band and applies tHe same to thereceiver stage and rejects signals of the second frequency band; andsecond circuit means coupling said winding of said transmitter filter tosaid antenna terminal whereby the transmitter filter selects signals ofthe second frequency band from the transmitter stage and applies thesame to the antenna terminal and rejects signals of the first frequencyband.
 2. The filter apparatus of claim 1 wherein said first circuitmeans includes capacitor means having a first plate connected to saidantenna terminal and a second plate connected to said receiver filter.3. The filter apparatus of claim 1 wherein said second circuit meansincludes capacitor means having a first plate connected to said antennaterminal and a second plate connected to said transmitter filter.
 4. Thefilter apparatus of claim 1 further including: first capacitor meanshaving a first plate formed by a portion of said winding of saidreceiver filter, a dielectric formed by a first insulating member whichis mechanically connected to said winding of said receiver filter, and asecond plate formed by a first conductive member having first and secondends, said first end being connected to said first insulating member andsaid second end being connected to said antenna terminal; and secondcapacitor means having a first plate formed by a portion of said windingof said transmitter filter, a dielectric formed by a second insulatingmember which is mechanically connected to said winding of saidtransmitter filter, and a second plate formed by a second conductivemember having first and second ends, said first end of said secondconductive member being connected to said second insulating member andsaid second end of said second conductive member being connected to saidantenna terminal.
 5. The filter apparatus of claim 1 wherein saidreceiver filter includes a plurality of cooperating helical resonatorswhich tend to have a spurious passband, each of said resonators having ahelical coil with first and second ends and a longitudinal axis, and ahousing member enclosing said helical coil which is formed by fourconductive sidewalls and first and second conductive end walls, saidlongitudinal axis of each of said coils being perpendicular to saidfirst and second conductive end walls; said resonators beingjuxtapositioned with respect to each other so that said housing memberof each resonator has a common sidewall with the housing member of acooperating resonator; said first end walls of said resonators beingformed by a single integral conductive member; means electricallyconnecting said first end of each of said coils to said integralconductive member; and each of said common sidewalls having an apertureformed therein, said apertures being located adjacent to said secondends of said coils to facilitate predominantly capacitive couplingbetween said adjacent cooperating resonators to provide attenuation tosignals having frequencies lying within said spurious passband.
 6. Thefilter apparatus of claim 1 wherein said transmitter filter includes aplurality of cooperating helical resonators which tend to have aspurious passband, each of said resonators having a helical coil withfirst and second ends and a longitudinal axis, and a housing memberenclosing said helical coil which is formed by four conductive sidewallsand first and second conductive end walls, said longitudinal axis ofeach of said coils being perpendicular to said first and secondconductive end walls; said resonators being juxtapositioned with respectto each other so that said housing member of each resonator has a commonsidewall with the housing member of a cooperating resonator; said firstend walls of said resonators being formed by a single integralconductive member; means electrically connecting said first end of eachof said coils to said integral conductive member; and each of saidcommon sidewalls having an aperture formed therein, said apertures beInglocated adjacent to said second ends of said coils to facilitatepredominantly capacitive coupling between said adjacent cooperatingresonators to provide attenuation to signals having frequencies lyingwithin said spurious passband.
 7. The filter apparatus of claim 1wherein each of said resonators has a helical coil with first and secondends, a coil form for supporting said coil and having first and secondends and an aperture extending from said second end into said coil form,and a housing member formed by four conductive sidewalls and first andsecond conductive end walls; said resonators of said receiver filterbeing juxtapositioned so that said housing members thereof have commonintermediate sidewalls; said resonators of said transmitter filter beingjuxtapositioned so that said housing members thereof also have commonintermediate sidewalls; said first end walls of said housing members ofeach of said filters being formed by a first integral conductive member,means electrically connecting said first ends of said coils to saidfirst conductive member; said second end walls of said housing membersof each of said filters being formed by a second integral conductivemember having a surface located in a plane which is parallel to andadjacent to said second ends of said coil forms; and a plurality oftuning means each extending through said second conductive member intosaid aperture of one of said coil forms, said plurality of tuning meansfacilitating simultaneous adjustment of said resonant frequencies ofsaid receiver and transmitter filters.